This invention relates to an electronic musical synthesizer having a preset circuit for disabling variable controls and enabling preset controls for filters, octave circuits and vibrato circuits.
Electronic musical synthesizers differ from electronic organs in that they include electronically tunable filters which change the harmonic content, or timbre, of the voice being artifically produced. Typically, the controllable filters comprise in cascade a bandpass filter, in which the bandwidth and center frequency are controllable, and a low-pass filter, in which the cut-off frequency is controllable. The tuning of these filters is changed continuously during the progression of a single tone, from its initiation to its decay. As a result, the sound emitted from the synthesizer speaker, during the time duration of a tone, is different from the sound produced when the tone was first initiated, unlike an electronic organ in which the tone remains continuous for any given voicing.
The timbre of the synthesizer voice is controlled by several variable devices which alter the frequency characteristics of the controllable filters. A variable Emphasis control changes the Q or bandwidth of the bandpass filter. A variable Color control changes the average amount of high harmonic content which is passed by the tunable filters, by changing the center frequency of the bandpass filter and the cut-off frequency of the low-pass filter. A variable Contour control changes the rate at which timbre is changed, or the amount of tuning change of the filters per unit of time. This is accomplished by adjusting the attack rate and decay rate of a contour envelope generator which controls either the center frequency adjustment for the bandpass filter, or the cut-off frequency adjustment for the low-pass filter.
An oscillator in the synthesizer generates an alternating or modulation signal, having a rate of frequency adjusted by a rate control, and a depth or amplitude adjustable by a depth control. A vibrato channel couples the oscillator to a tone combining circuit to create a vibrato effect in similar fashion as in an electronic organ, in that the alternating signal varies the frequency of the basic tone being generated. Unlike an electronic organ, in which tremolo is an amplitude change of tone, a synthesizer tremolo effect relates to a frequency spectrum change. The synthesizer tremolo channel couples the oscillator to the Color control portion of the synthesizer.
By adjusting these frequency related controls, the operator of an electronic musical synthesizer can accomplish a great variety of novel musical effects not possible with conventional musical devices. In addition, the operator can imitate known musical instruments more closely than is possible with other types of electronic instruments. Because it is difficult foro an inexperienced operator to arrange these controls in the precise manner necessary to imitate well-known instruments, it has been conventional to provide a switching circuit with voicing tabs for each musical instrument to be imitated. upon selecting a particular musical instrument by depressing the associated tab switch, a resistor matrix passes preset control voltages to the controllable filter.
However, such preset control voltages can be altered by the setting of the various frequency related controls described above. While the ability to alter is desirable to an experienced operator, in order to create unique variations upon known instruments, it is very difficult for an inexperienced operator to set all of the frequency related controls to a null position when attempting to simulate a known instrument. The presence of several controls, each of which must be moved to a zero setting, and proper setting of the octave controls and other adjustable devices present an overwhelming number of variations to an inexperienced operator. In addition, slippage in the mechanical linkage for the knobs, slides and switches, backlash have made it difficult to ensure optimum simulation of well-known instruments unless the operator has an experienced ear for detecting minor variations which can occur inadvertantly.